In recent years, techniques for implementing a high-speed wireless communication have been researched and developed in a wireless communication field. High Speed Downlink Packet Access (HSDPA), which is a high-speed data communication technique in Wideband Code Division Multiple Access (W-CDMA), is one example of such techniques. In HSDPA, Adaptive Modulation and Coding scheme (AMC) is employed, and a communication rate at a maximum of 14.4 Mbps is achieved in a case where a state of radio wave propagation is satisfactory. AMC is a technique for adaptively controlling a modulation method and a coding rate of an error correction code according to the state of radio wave propagation from a wireless base station device as a transmission device to a terminal device as a reception device. To implement such a high-speed wireless communication, it is needed to prevent reception performance from being degraded. It is desired to prevent reception performance from being degraded, for example, by a multi-path. A multi-path is a communication failure where a signal transmitted from a transmission device propagates through a plurality of paths (transmission channels), become a plurality of signals (multi-path signals) having different timings, and reach a reception device due to a phenomenon that a radio wave is refracted or reflected by being affected by a building, a terrain or the like.
Techniques for preventing reception performance from being degraded by a multi-path include Chip Correlation Minimum Mean Square Error (MMSE) Receiver with Multi-path Interference Correlative Timing (MICT) (CCMRM). CCMRM is one of techniques for removing a distortion of a reception signal, which is caused by a multi-path, by using an equalizer such as a Finite Impulse Response (FIR) filter.
In CCMRM, a weight coefficient is obtained such that an interference component is canceled and a signal component is increased by using a correlation among a plurality of paths caused by a multi-path. Specifically, a weight coefficient is obtained with the following computation method by using a correlation matrix that represents a correlation among paths, and a channel estimation value of each of the paths.
A case where reception diversity is OFF, namely, a case where the number of reception antennas included in a reception device is one is illustratively described. In this case, a value of each of components Rij, which configure a correlation matrix R, is computed with the following expression (1).
                              R                      ij            ⁢                                                                =                                            ∑                              n                =                0                                      255                    ⁢                      v            ⁢                                                  ⁢            1            ⁢            v            ⁢                                                  ⁢                          2              *                                                          (        1        )            
Here, i and j are finger numbers of fingers that respectively correspond to a specified number of paths among a plurality of paths caused by a multi-path. i and j are, for example, 15 integers from 0 to 14. v1 is input data before being despread per chip proceeding from a reference chip by i, v2 is input data before being despread per chip proceeding from the reference chip by j. m is the number of added samples per chip. In the example represented by the expression (1), m is 256 chips (1 slot). Moreover, a symbol “*” represented in the expression (1) indicates a complex conjugate.
As represented by the expression (1), a multiplication of v1×v2* is performed per chip, and multiplication results of 256 chips are added. Moreover, a moving average of a fixed section (for example, 10 slots) is obtained for a result of the computation obtained with the expression (1), and a value of each of components Rij, which configure the correlation matrix R, is computed.
A weight coefficient Wi is computed with a Gauss-Seidel iterative method by using the computed value of the correlation matrix component Rij and a channel estimation value Hi as represented by the following expression (2).
                                                        w              ⁡                              [                n                ]                                      ⁡                          [              t              ]                                ⁡                      [            i            ]                          =                              (                                                            H                  ⁡                                      [                    t                    ]                                                  ⁡                                  [                  i                  ]                                            -                              (                                                                                                                                                          ∑                                                          j                              =                              0                                                                                      i                              -                              1                                                                                ⁢                                                                                                                    R                                ⁡                                                                  [                                  i                                  ]                                                                                            ⁡                                                              [                                j                                ]                                                                                      ×                                                                                                                            w                                  ⁡                                                                      [                                    n                                    ]                                                                                                  ⁡                                                                  [                                  t                                  ]                                                                                            ⁡                                                              [                                j                                ]                                                                                                                                    +                                                                                                                                                                          ∑                                                      j                            =                                                          i                              +                              1                                                                                                            N                            -                            1                                                                          ⁢                                                                                                            R                              ⁡                                                              [                                i                                ]                                                                                      ⁡                                                          [                              j                              ]                                                                                ×                                                                                                                    w                                ⁡                                                                  [                                                                      n                                    -                                    1                                                                    ]                                                                                            ⁡                                                              [                                t                                ]                                                                                      ⁡                                                          [                              j                              ]                                                                                                                                                                          )                                      )                                                                                R                  ⁡                                      [                    i                    ]                                                  [                ]                            ⁢              i                        ]                                              (        2        )            
Here, t is a transmission antenna number of each of transmission antennas included in a transmission device. t is, for example, two integers from 0 to 1. H[t][i] is a channel estimation value Hi for each of the transmission antennas. N is a maximum number of fingers. N is, for example, 15. n is an iterative number of times of the computation represented by the expression (2) according to the Gauss-Seidel iterative method. n is, for example, three times from n=0 to n=2. w[n][t][i] is a weight coefficient wi at an nth time, which is iteratively computed, and is a weight coefficient wi for each of the transmission antennas. In the case of n=0, namely, in the initial computation of the expression (2), “0” is set as an initial value of w[n][t][i] used for the computation. Moreover, in an iterative computation performed in the case of n>0, namely, in a computation performed at and after the second time, the value of w[n][t][i] precedingly computed is used as the initial value.
The weight coefficient Wi is obtained from the weight coefficient w[n][t][i] obtained with the computation performed by a specified iterative number of times. For example, if the number of transmission antennas is 2 and the iterative number of times is 3, weight coefficients of the two transmission antennas are obtained respectively with the following expressions (3) and (4).W[0][i]=w[2][0][i]*  (3)W[1][i]=w[2][1][i]  (4)
The weight coefficient Wi obtained with the expressions (1) to (4) is set as a tap coefficient of an FIR filter. The FIR filter includes n (such as 15) delay circuits for delaying input data by unit time (1 chip), n multipliers for respectively multiplying output data of these delay circuits by a corresponding tap coefficient Wi, and an addition circuit for adding output data of these multipliers. Reception data input to the FIR filter is delayed by each of the delay circuits, and n data are generated inclusive of undelayed reception data. Each of the generated data is multiplied by a corresponding tap coefficient Wi. Each of the data multiplied by the tap coefficient Wi is added (summed up) by the addition circuit, and the added data is output from the FIR filter.
According to the above described CCMRM, a distortion of a reception signal, which is caused by a multi-path, can be removed, whereby reception performance can be improved. However, a complex multiplication between a correlation matrix component Rij, which is a complex number, and a weight coefficient wi, which is a complex number while being iteratively computed, is performed when the weight coefficient Wi is computed as represented by the expression (2). A complex multiplication performed once is implemented by real number multiplications performed by four times and additions performed by twice. In a logic circuit, the processing amount of a multiplication process is large. Therefore, the length of processing time and power consumption increase as the number of times of multiplications grows. [Prior Art Document] T. Hasegawa, M. Shimizu, “A chip Correlation MMSE Receiver with Multiple Interference Correlative Timing for DS-CDMA systems” Proc. IEEE Veh. Tech. Conf. (VTC 2005 spring)